More than 20 MPs are claiming taxpayer-funded expenses to rent homes in London whilst simultaneously letting out property in the capital, it emerged last night.

They can claim up to £20,000 a year each for rent as well as receiving rent from properties that were often purchased and refurbished from taxpayers' money.

The disclosures come as the Independent Parliamentary Standards Authority (Ipsa) confirmed on Thursday at least eight MPs are either letting properties to, or renting from, another MP. Critics have claimed that is a loophole in the rules that allows them to continue building up capital with public money.

Examination of the register of members' interests shows that those who are renting a London home whilst claiming rental income include Liam Fox, the former defence secretary and the former ministers Peter Luff and Nick Harvey. None have broken parliamentary rules. John Bercow has attempted to prevent the expenses regulator from disclosing the identities of MPs landlords on the grounds that it may put MPs' safety at risk.

Sources at Ipsa said there were genuine concerns that it would be possible to identify MPs' homes if their landlords' names were released.

Some MPs believe that by using the internet and the land registry, it may be possible to work out where they live. That is being investigated by Ipsa.

Critics, including the campaigning Labour MP John Mann, have argued that some information should be released which would allow the public to work out which MPs are renting to each other.Bercow's letter has echoes of the attempt by the previous Speaker, Lord Martin, to prevent the release of addresses on which parliamentary expenses were claimed. The information was subsequently sold to the Daily Telegraph, causing the scandal that led to the resignation or prosecution of dozens of MPs. Martin had to resign after fighting to keep information about expenses secret.

Ipsa banned MPs from claiming back mortgage interest after a transitional period, which ended this summer. That followed concerns that politicians were able to build up valuable property empires at taxpayers' expense.

Its initial rules banned MPs from renting properties from family members, close business associates or "an organisation in which you or a family have an interest". That was to help ensure that MPs did not claim the market value for rent from the taxpayer while actually paying far less

The rules were later clarified to allow one MP to rent from another, provided they were not related or married. It is thought that rule was specified after requests from MPs.

There have been increasing concerns over MPs' security since Stephen Timms, the Labour MP for East Ham, was stabbed in his surgery by the Islamist Roshonara Choudhry in May 2010.

Some politicians have said their homes have been identified by extremists and been subjected to attacks. Sajjad Karim, Tory MEP for the North West, said last year his family felt threatened by English Defence League members who congregated outside his home.